This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "alveolar_capillary_barrier_failure#Alveolar Epithelial and Endothelial Junctional Disruption"). Disorder-specific substitutions at the trigger node: ARDS substitutes a direct pulmonary or indirect systemic insult; high-altitude pulmonary edema substitutes sustained alveolar hypoxia with HIF-1-alpha signaling; influenza and tuberculosis substitute alveolar pathogen replication and the ensuing immune response; COPD substitutes repeated whole cigarette smoke exposure; inhalational injury substitutes an oxidant gas or particulate; and lung transplantation substitutes cold-storage ischemia-reperfusion. The conserved central cell types are the pulmonary alveolar type 1 and type 2 cell and the pulmonary capillary endothelial cell.
Much of the direct human evidence for the trans-barrier propagation step comes from microphysiological alveolus-on-chip systems rather than from patients, because separating the epithelial from the endothelial compartment while exposing only one of them is not achievable in vivo. That is a strength for establishing the coupling and a limitation for establishing its magnitude in human disease; conforming entries should keep the evidence_source distinction visible rather than presenting chip results as clinical observations.
This module is deliberately narrower than `epithelial_barrier_dysfunction`, which models the allergic / type-2 "epithelial barrier hypothesis" across skin, airway, gut, and esophagus and terminates in IgE sensitization and the atopic march. That module is about allergen penetration driving sensitization; this one is about blood-gas barrier mechanics driving edema and gas-exchange failure. A disorder may legitimately conform to both.
Alveolar-Capillary Interface Insult
trigger
An insult is delivered to the alveolar surface or to the capillary side of the blood-gas barrier. The initiating agents are heterogeneous — oxidant gases, airborne particulates, whole cigarette smoke, replicating pathogens, sustained alveolar hypoxia, and cold-storage ischemia-reperfusion — but each converges on oxidative stress and innate inflammatory signaling at the interface. This node is the disorder-specific entry point; conforming entries substitute their own agent here and inherit the chain below.
Downstream
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Alveolar Epithelial and Endothelial Junctional Disruption
Oxidative and inflammatory injury at the exposed surface directly destabilizes the junctional complexes that give the barrier its selectivity.
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Cyclic Mechanical Strain Amplification
An injured barrier is subjected to the same continuous breathing-associated deformation as a healthy one, so the insult is delivered into a mechanically loaded tissue rather than a static one.
Cyclic Mechanical Strain Amplification
amplifier
Breathing subjects the alveolar wall to continuous cyclic deformation. Within the physiological range this strain maintains barrier and surfactant function, but supraphysiological strain — from mechanical ventilation, hyperinflation, or vigorous spontaneous effort — amplifies an established insult rather than initiating one on its own. Strain increases apparent permeability, potentiates pathogen translocation, and augments profibrotic signaling in the injured alveolar wall. This node is a modifier of severity and is why the same insult produces different barrier outcomes in a spontaneously breathing and a mechanically ventilated lung.
Downstream
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Alveolar Epithelial and Endothelial Junctional Disruption
Supraphysiological cyclic deformation of an already injured alveolar wall accelerates junctional loss and raises measured barrier permeability.
Alveolar Epithelial and Endothelial Junctional Disruption
central effector
Tight and adherens junction complexes are lost on both faces of the barrier — ZO-1 and occludin between alveolar epithelial cells, VE-cadherin and PECAM-1 between capillary endothelial cells — with visible intercellular gap formation, falling transepithelial electrical resistance, and shedding of soluble adhesion molecules into the alveolar and vascular compartments. This is the rate-limiting, disorder-agnostic step: whatever the initiating insult, the barrier fails here, and this is the node conforming disorder entries should target.
Downstream
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Trans-Barrier Injury Propagation and Increased Permeability
Loss of junctional integrity is what makes the barrier permeable, and the shared basement membrane is what allows the loss to spread from the injured layer to the uninjured one.
Trans-Barrier Injury Propagation and Increased Permeability
effector
Permeability of the blood-gas barrier rises, and injury crosses it. Exposure confined to the alveolar epithelial surface produces measurable injury in the capillary endothelium that was never directly exposed, and the reverse coupling holds for insults delivered to the vascular side. Propagation is mediated by soluble mediators, oxidative signaling, and adhesion-molecule shedding acting over short distances across the fused basement membrane rather than by direct contact of the agent with the second layer. The functional consequence is a barrier that passes protein and fluid.
Downstream
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Protein-Rich Alveolar Flooding and Gas-Exchange Failure
Once the barrier passes protein, oncotic restraint on fluid movement is lost and the interstitial and alveolar compartments flood.
Protein-Rich Alveolar Flooding and Gas-Exchange Failure
consequence
Protein-rich fluid accumulates in the interstitium and airspace. Flooded and collapsed alveoli continue to be perfused but no longer ventilated, producing shunt physiology, hypoxemia, and reduced compliance. Surfactant is diluted and inactivated by the exuded protein, lowering the surface-tension threshold for further collapse and closing a positive-feedback loop back onto the barrier. Whether the process resolves through epithelial repair and active fluid clearance or progresses to fibroproliferative remodeling is determined downstream and is disorder-specific.